Robatoy wrote:
On Mar 23, 10:50 am, "RicodJour" wrote:
In other words the steel supports should be
designed to take _all_ of the load so that the granite can do what it
is there for - look pretty - without being required to be structural
as well.

Correct. Now apply that same standard to a plywood substrate. Just to
clad the bottom of a slab of granite would be a waste of time. If 3/4"
stone (either e-stone (quartz-based) or g-stone (genuine granite) is
used, one MUST build a support which on its own will support the load.

I use assorted engineering programs for figuring deflection and such.
With a 350 pound load on 3/4" plywood on a 24x24 base, the deflection
is less than a 1/16th. You are conservative and I'd never dream of
telling you that was a bad thing.

The 3/4 stones are NEVER allowed to take a structural role in a
cantelevered application.

Right. In that link I posted some of the replies indicated it was
possible to cantilever the stone by itself. That's a bad idea until
you get into thick stuff, 2"+ depending on the stone, which would not
be suitable for a countertop.

'Real' granite is full of fissures and can
snap at any time... the risk of 'fissures' is virtually non existant
in e-stone. A 12" overhang is safe in 1 1/4" e-stone..ASSUMING the bar
top is anchored in such a way that it can't tip on a fulcrum further
away than 1/3 from the load. (That is a guide-line.) If the top is 18"
wide, I would spec a support at 6" and 12" along it's length no
further than 24" apart.
The support at the opposite edge from the load can be closer than 6"
from the edge.

The 1/3 cantilever rule of thumb is fine for wood joists and such, but
it's misleading as soon as you change materials or have dissimilar
materials trying to act in concert. I pointed that out to someone and
they got mad.

Robatoy's suggestion of using a thicker, engineered (man-made) stone
with let-in steel is certainly a good way to go, but it is also
frequently a more expensive way to go as letting into the stone
requires more labor and entails more risk.

Any stone shop worth its salt can let in a 1/2" thick metal bracket
with sufficient radii along the sides and end of the slot to minimize
stress-risers.

With just that one comment you've made it clear you know what you're
talking about. I don't doubt that you'd do a fine job letting in for
a brace/bracket. It's not _you_ I'm worried about. You and I both
know that some "stone" guys would hog out a let in brace with their
wet skilsaw and knock out the pieces with a hammer - no one would ever
see it, right? In general, most people asking the question about how
to support a stone cantilever wouldn't know what to avoid. You do.

The thicker material won't require a glued-on (visible
line) edge and an added edge in a flexing situation like a bar top is
not a good idea. All the dicking around to properly support a 3/4"
slab will evaporate the savings in materials quickly.

I thought we agreed that there should be no flexing in the stone at
all.
Out of curiosity, what's your ballpark percentage upcharge going from
3/4" with a laminated edge to 1 1/4" granite?

So, even though one is not supposed to count on a structural component
in a floating application, it sure is nice when the strength is there
to begin with.

Okay, we did agree. Thought I was losing my mind for a second! I
can't argue with you there.

The thicker stone adds a
lot more weight to the countertop, which may or may not be a problem
structurally, and you're paying for that extra stone that you'll never
see. The engineered stone also limits the choices you'll have in
stone.

The increased weight is a worthwhile investment to get the strength
and stiffness. Not to mention a much better look.

You may be right, but what's your opinion on the suitability of the
OP's island with respect to his floor capacity? Right - I don't know
either. When I hear of an L-shaped 8' x 11' island, I think of
possibly exceeding the floor's design load. Personally, I think the
OP shouldn't go a step further until he determines what the floor is
designed for and how this, presumably fully loaded, island will affect
deflection and bounce. Your 1 1/4" engineered stone top alone would
weigh almost 550 pounds and that's on only 34 SF of floor. The
standard design load, live and dead loads, for a first floor is on the
order of 55 PSF. 15 PSF for the structure, ~15 PSF for your
countertop, leaves only 25 PSF reserve for cabinets and contents,
appliances and people. That island could _easily_ exceed the
allowable floor load. At the very least you'd have lots of deflection
and a bouncy floor.

In granite, 3/4" is a non-starter. An invitation to disaster in a
suspended bar application. As I mentioned before, the investment to
properly support that brittle **** would be way more money than an
upgrade to 1-1/4"

It sounds like you're a dedicated stone installer. I'm not. I gotta
buy the stuff, and it's easier and cheaper for me to cut the plywood
and add some steel bracing as required. Of course I am not locked
into the plywood as any such decision must be based on the stone used.

I have 21 choices in my granite palette, but 60 in Quartz. The
uniformity of the patterns and non porous surface AND it's raw
strength make for a far more durable surface than granite.
When hit with a very hot pan, the possibilty of moisture in granite
can create quite a catastrophic failure.

No argument about the homogeneity and overall durability of the
engineered stone, but some people _like_ the random swirls and
patterns of real stone. And face it, no one's ever come to you and
said, "I don't care how it looks, I just want it to last forever."
The choice of stone vs. laminate or Corian is an aesthetic decision.
I can't tell people what to like. I can only warn them of the
consequences, and in that we're on the same page.

I'm kind of surprised that you only have 21 natural granites. Since
China and India opened up the floodgates the choices are almost
unlimited. Are the 21 granites your way of keeping your life simple
and not overwhelming customers?

The idea is to add strength and stiffness. Cutting into a
thin slab creates stress concentration points, and let-in braces are
weakening the stone in order to strengthen it - that doesn't make
sense to me.

Doesn't to me either. The letting in of a bracket will only be doable
in 1-1/4" e-stone material.

R's comment about idiots doing stupid things on stone cantilevered
countertops should not be discounted. I once swung a hammer at an
idjit painter's ankles that was _standing_ on the cantilever! He
jumped off, and started yelling at me, and I told him that the only
reason I didn't swing at his head was because I couldn't reach it.

Cantilever's are one of the few things where I tell people to
purposefully over-build things, as deflection and stiffness are of
paramount importance. Doubly so with a stone countertop.


When using 3/4". make sure that clown can tapdance on the substrate
before you apply that thin skin of granite. 3/4" is for low-cost
vanities.

It flashed across my mind when the idjit painter was up there that I
didn't want to hit his toes, which was my first thought. I figured
he'd pull his foot away and I'd bust up the countertop, so I swung at
his ankles.

R